Audio Channel Transport Over TSIP
The Telecom Serial Interface Port, TSIP, implemented on Shannon (and Tomahawk) is
optimized for use with telephony infrastructure backplanes such as H.110, H-MVIP and
SC-BUS that contain multiple, synchronous lanes carrying data octets. These use clock
rates that are a multiple of 2.048MHz and frame sizes that are multiples of 32 bytes.
Specifically, it is documented for use with the three clock rates of 8.192MHz,
16.384MHz and 32.768MHz with the associated frame sizes of 128, 256 and 512 bytes
per lane. However, the TSIP is not limited to these rates and frame sizes. It can
interface with any synchronous serial data stream that places the same number of bytes
in each frame. The clock rate can be optimized for the data throughput required
because the interface is fully synchronous.
Suppose that you needed to transport 4 stereo channels of standard CD audio. The
sample rate is 44,100 samples per second. The data samples are 16 bits each. The 4
stereo channels are actually 8 discrete channels. Total throughput is 5,644,800 bits per
second. If we divide the 44,100 samples per second by 8 we can get a frame rate of
5,512.5 pulses per second. Each frame will contain 8 samples for each discrete stream.
They can be interleaved to minimize delay or the data for each stream can be in blocks.
In this example, you still have 128 bytes in each frame (4 stereo channels, 8 samples
per frame and 2 bytes per sample). The standard configuration for 8.192MHz would
work simply by providing the TSIP with a 5,644,800Hz clock.
The TSIP would work equally well if the frame size had to change. If we use the
example above but we only need 3 stereo channels. There are two solutions. The first
is to use the configuration described above. In this case, you simply have unused
timeslots in your data interface. This is acceptable as the software will know the
quantity of active channels and the unused timeslots to not consume any additional
CPU cycles. The second solution allows for the clock rate to be reduced and the frame
size to be reduced. The frame length will now be set to 96. The frame rate could still be
the same 5512.5 pulses per second. Lastly the clock rate would be 4,233,600Hz.
These same kinds of calculations can be applied to larger numbers of channels. The
TSIP interface supports 8 lanes of data with up to 128 bytes per frame, 4 lanes of data
with up to 256 bytes per frame and 2 lanes of data with up to 512 bytes per frame. The
aggregate in all of these modes is that the TSIP can support up to 1024 bytes per frame
period. So, if we had to transport 32 channels of stereo CD audio instead of only 8 like
the first example, the same configuration can be used with 4 stereo channels on each of
the 8 lanes.
Please note that these throughput values are all bi-directional. There are separate RX
and TX pins for each TSIP.
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